1. Field of the Invention
This invention relates to photographic fast wide angle objectives in which the total length of the lens system is nevertheless short and compact.
2. Description of the Prior Art
In order to minimize the bulk and size of the camera, a demand has arisen for a compact photographic objective whose length from the first lens surface to the focal plane, or the total length of the lens system, is short. Particularly desirable is an objective with a lens system whose total length is in the order of its focal length. To obtain such a lens system, it is expedient to make the front group of the lens system of a positive refractive power and the rear group of a negative refractive power, or to employ a so-called telephoto type arrangement. The telephoto type of arrangement has been used in many photographic objectives of long focal lengths with relatively narrow angular fields, but has found very little use in objectives of wider than standard angular fields. The reason is that with the power distribution of a telephoto type arrangement, as the angle of the view field increases, distortion and astigmatism increase, and they become difficult to correct.
Japanese Pat. Sho Nos. 44-10831 and 52-48011 propose photographic objectives of the telephoto type which attempt to increase the angular field. The former has a relatively narrow angular field of about 46.degree., and the latter, though having a relatively wide angular field at 63.degree. is slow, with an F-number of 4.5.
On the other hand, in telephoto type wide angle objectives, the positioning error of the front lens group in a direction perpendicular to the optical axis, or so-called parallel decentering error, causes an extraordinary decrease in the imaging performance. In the paraxial region particularly, uniform coma is produced, and the intermediate zone of the field has an unsymmetrical blur, thus extraordinarily lowering the image quality over the entire area of the picture frame. In the telephoto type of lens system, therefore, it is of great importance to choose a lens configuration and a refractive power distribution which allows for the assembling error to influence the result as little as possible.
Other prior techniques of interest of the invention of the present patent application appear in Japanese Laid Open Pat. Sho Nos. 50-145226 and 50-145227 and Japanese Pat. Sho No. 56-50248.